Mineralogical Controls on Microbial Reduction of Fe(III) (Hydr)oxides

This chapter discusses the current state of knowledge regarding the geochemical controls on the initial and sustained reduction of Fe(III) minerals. It introduces the major controlling factors for microbial Fe(III) reduction. Cultured model dissimilatory metalreducing bacteria (DMRB) show diminished abilities to reduce the more crystalline phases, only reducing a small fraction of the potentially available Fe(III). The chapter summarizes the current understanding of the geochemical and mineralogical constraints on microbial Fe(III) (hydr)oxides reduction with the realization that the field is far from a resolution or consensus on what factors ultimately control the transfer of electrons to Fe(III) mineral surfaces. Recent investigations have clearly illustrated differential expression of proteins involved in metal respiration in the common DMRB Geobacter sp. More sophisticated techniques and experimental approaches, including surface-sensitive and time-resolved spectroscopy need to be used to accurately interrogate the molecular environment. The use of poised anodes as surrogate minerals holds promise in addressing the role of reduction potential in electron transfer processes. Ultimately, unraveling the enigma of the microbe- mineral interface will require a multidisciplinary approach requiring an appreciation of the physics and chemistry of mineral surfaces, the enzymatic and nonenzymatic pathways responsible for electron transfer, and the ecology of metal-reducing microbes within a complex mineral framework.